1. Field of the Invention
The present invention relates to a sensor control apparatus and a sensor control system which detect humidity within a gas under measurement using a gas sensor for detecting a particular gas component contained in the gas under measurement.
2. Description of the Related Art
One of conventionally known sensors used in automobiles is an oxygen gas sensor which is attached to an exhaust passage of an internal combustion engine (e.g., an automotive engine) so as to detect the oxygen concentration of exhaust gas. This oxygen sensor detects the oxygen concentration and thus detects the air-fuel ratio of the exhaust gas by making use of a phenomenon in which the magnitude of current flowing through a sensor element changes with the oxygen concentration of the exhaust gas. A sensor control apparatus which drives the oxygen sensor has a function of energizing and controlling the sensor element, and converting the current flowing through the sensor element to a voltage and outputting the voltage to an electronic control unit (ECU). The ECU obtains the oxygen concentration or air-fuel ratio of the exhaust gas based on an output from the sensor control apparatus. In the ECU, the oxygen concentration or air-fuel ratio of the exhaust gas thus obtained is utilized for air-fuel ratio feedback control such as adjustment of fuel injection amount.
The characteristics of an oxygen sensor mounted on an automobile can possibly change as a result of, for example, deterioration with time or normal wear and tear wherein the sensor output shifts even under the same conditions. In order to solve such a problem, a known technique corrects a change in the characteristics of the oxygen sensor based on the sensor output in an air atmosphere. Also, an air-fuel ratio detection apparatus has been disclosed which detects the humidity of the atmosphere using an oxygen sensor, and renders the correction of the sensor characteristics more accurate in accordance with the humidity (see, for example, Patent Document 1). Here, specific methods for detecting the humidity using an oxygen sensor will be described. In one method in which an oxygen concentration cell utilizing a solid electrolyte body is used, the voltage of an oxygen pump is switched alternatingly between a first preset voltage at which electrolysis of moisture within a gas under measurement does not occur and a second preset voltage at which electrolysis of the moisture within the gas under measurement occurs. The moisture concentration is then determined from the difference between the oxygen pump currents measured in the two cases. In another method in which a two-cell-type oxygen concentration cell is used, the control voltage of the oxygen concentration cell is selectively set to a first preset voltage and a second preset voltage, the oxygen pump currents in the two cases are measured, and the moisture concentration is determined from the difference between the oxygen pump currents (see, for example, Patent Document 2).
[Patent Document 1] Japanese Patent No. 5021697
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 563-85351